tag.lua: fix new client screen

Prevent from blinking the first screen in multiheads by using
the mouse-focused screen tag list for a new client instead of
screen 1.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
Signed-off-by: Julien Danjou <julien@danjou.info>

lib/awful/tag.lua.in

@@ -316,7 +316,7 @@ function withcurrent(c, startup)
             c:tags(c.transient_for:tags())
         end
         if #c:tags() == 0 then
-            c:tags(selectedlist(c.screen))
+            c:tags(selectedlist(capi.mouse.screen))
         end
     end
 end